A primary school teacher who abused scores of pupils over 13 years continued teaching for 11 years after an allegation was made against him, it emerged yesterday.

John Walker, 58, resigned from a state primary school and moved to the independent sector after his local education authority received an anonymous letter in 1989 alleging sexual abuse by him.

Oxfordshire issued him with a formal warning, but failed to inform the police or the education department.

Rye St Antony, the private school where he subsequently worked and to which no charges relate, ran standard checks with the department and with police, but there was nothing to indicate he should not be employed.

He continued teaching nine to 11-year-olds at the school up until last November, when a victim from the state school, who had confided in his girl friend, came forward and police began an investigation.

Yesterday Walker pleaded guilty at Oxford crown court to 35 charges of indecent assault and gross indecency against 12 children, six boys and six girls, between 1976 and 1989. He denied a further 20 charges, relating to six boys and one girl; the judge ordered these to lie on file; police are continuing with their investigation, and believe he may have had as many as 60 victims.

The court heard that Walker, single, of Headington in Oxford, would pick "favourite" pupils and assault them in his classroom, in his car, in changing rooms for swimming lessons, or at his home.

He ran extracurricular chess and football classes, giving him further access. He also helped out at annual summer camps, at which he would abuse his victims, aged eight to 16, during midnight walks through graveyards or after sneaking into their dormitories and putting his hands down their sleeping bags.

Sandy Stansfield, prosecuting, told the court: "One victim recalls Mr Walker being superficially friendly and having favourite pupils. Others he used to bully and humiliate."

One boy, who claimed to have been assaulted at least 30 times, was told by Walker: "This is not hurting anyone - it's only society that thinks this is is wrong; there is nothing wrong with doing this."

A girl aged 10 was told "this will make your headache better", as Walker put his hand up her dress. Another 10-year-old suffered for a year. "The abuse consisted of Walker fondling her breasts," Ms Stansfield explained. "He would make an excuse to get her out of the classroom and into the corridor. Every time they had PE he would touch her."

Several of his victims and their families were in court as Walker pleaded guilty to each charge, The victims' testimonies, read out in court, pointed to them suffering prolonged psychological damage while "trapped in a cycle of abuse".

After the hearing Detective Constable Steve Hart of Oxford CID said Walker's guilty pleas had avoided victims suffering the trauma of giving evidence: "For a long period he preyed on young children and was a danger to them. We are very pleased with the result."

John Mitchell, a spokesman for Oxfordshire education department, said no records now existed to show a complaint had been made against Walker, but that, if it had, "it would have been incumbent on us to tell the education department, who would have told the police".

Alison Jones, the headteacher of Rye St Anthony, said: "Prior to hiring him we carried out our usual checks with the department and the police, and there was nothing at all to indicate anything untoward."